1. Field of the Invention
The present invention generally concerns electronic wattmeters as are commonly used by electrical utilities in the monitoring and reporting of electricity usage by homes and by businesses.
The present invention particularly concerns electronic wattmeters that have no current transformer for the sensing of current consumption, and that report measured and monitored electricity consumption (i) by radio (ii) on demand (iii) in relays to a base station.
2. Description of the Prior Art
2.1 General Requirement Circa 1990 for An Electronic Power Meter
U.S. electrical utilities have desired for some time power meter features and capabilities that the traditional electromechanical power meter cannot provide. However, these desires have not as of yet, circa 1995, been considered sufficient justification for a complete one-for-one replacement of existing residential power meters.
Without the market scale which would be dictated only by wholesale replacement of existing residential power meters, electronic meters have been relegated to niche markets. Now (circa 1995), however, electrical utilities face the new requirement of wheeling. With the advent of Wheeling, utilities *are willing to consider a complete replacement of the existing electromechanical meters. It is desired that a more powerful electronic meter would be produced at a price comparable to, or less than, the existing electromechanical meter.
Utilities throughout the United States recognize that Wheeling is rapidly becoming a reality and the resulting competition will dictate the adoption and utilization of a new metering technology. Many new and interesting metering products are becoming available, but to date these products have targeted small niche markets. The price and longevity of the electromechanical meters have prevented direct competition for the large residential market. However, wheeling [defined and further discussed in the fourth paragraph hereinafter] now provides the opportunity to upset this long-standing impasse.
The requirements for electric utility metering in the United States are driven by the customer. There is a general concern for customer satisfaction in the utility industry which is being driven to some degree, circa 1995, by the prospect of wheeling. The specific areas where utilities are interested in improving their customer interface are in offering (i) variable rates for load management, (ii) direct load curtailment, (iii) time of use (TOU) information, (iv) security, and revenue protection, (v) reduced expenses in meter reading, and (vi) customer interactive communications.
At the present time there appears to be an abundance of reasons for the U.S. utility industry to anticipate changes and prepare to operate competitively in a significantly different business environment. Some of the factors that contribute to the changes are include: (i) the present and long term United States economics trend of expansion, (ii) wheeling, (iii) electric vehicles, and (iv) new technology.
Detailed discourse regarding the US economy is beyond the scope of this disclosure. However, U.S. electric utilities have been seriously affected in the past by varying interest rates. Power generation expansion that requires borrowed capital could put a utility at risk.
Wheeling became a reality when the Public Utility Regulatory Policies Act of 1978 (PURPA) amended the Federal Power Act and gave the Federal Energy Regulatory Commission (FERC) expanded responsibilities for the encouragement of co-generation and small power production using alternative energy technologies. See Electric Power Wheeling and Dealing, Technological Considerations for increasing Competition; Congress of the United States Office of Technology Assessment PP 55. The goals of PURPA were to advance: 1) conservation of electric energy, 2) increased efficiency in electric power production, and 3) achievement of equitable retail rates for consumers. This was advanced in large part by requiring utilities to interconnect with and buy power from co-generators and small power producers that met standards established by FERC. This was the first major Federal move to open up electricity markets to non-utilities.
There have been other amendments to the Act of 1978 and all the utilities that were contacted in our market survey are preparing for the added competition that the act allows. In this relative new competitive environment, the energy will be billed to the utility that can provide the best service at the lowest cost. Better service can be accomplished by: 1) instantaneous readouts for shut-offs, turn-ons, verification, etc. 2) TOU (time of use) profiles which will allow rate change allowances, 3) faster and more accurate billing and 4) demand load control. Finally, the lower cost will result from: 1) manual meter reading elimination, 2) load leveling, and 3) improved customer relations.
Another government mandate that is destined to impact the utility industry is electric vehicles. Air pollution has prompted the State of California to establish quotas for non-polluting (electric) vehicles in the Los Angeles area. Not only will the use of electric vehicles increase the need for electrical energy, but a new metering concept will be required. At least one utility is currently buying communications equipment so that each electric vehicle can have it's own individual power meter. This allows the utility company to appropriately charge the vehicle owner irrespective of where the vehicle batteries are recharged. The magnitude of the added energy usage requirement is clearly brought into perspective in realizing that the energy requirements for an electric vehicle would about parallel the usage of a typical residence.
It is interesting and important to note that the DRM system proposed here would exactly satisfy the electric vehicle metering requirements. A readout could be accomplished at any time when the vehicle is domiciled at a designated location. Alternatively a search and read program could be implemented to perform a readout anywhere in the utility service area.
A number of advancements in new technology can now provide the basis for a significantly different residential power meter. Microprocessors have been developed for large volume applications, particularly for the automotive industry, and RF integrated circuits have been developed for large volume application such as commercial radios, pagers and portable telephones. The large volume is the ingredient that dictates the economics, and these particular components are directly applicable to the power meter application.
2.1 General Obiectives of a New Electronic Power Meter
An increased-capability electronic power meter should be provided at a price comparable to the existing residential electromechanical power meter. However, capabilities will be enhanced. The desired new capabilities include the following:
A new electronic power meter would desirably offer built-in two-way RF communications for automatic remote meter reading.
It would retain hourly time-of-use (TOU) measurements for 30 days. Alternatively, it would retain TOU measurements every 21/2 minutes for 2 hours,
A new electronic power meter would desirably offer record the time of, and send an alarm, at the onset of any tampering so as to help preclude energy theft.
It would desirably provide a remote display for customer communications.
Finally, a new electronic power meter would desirably provide output control for load curtailment.
The new electronic power meter should have an operational life of at least 15 years with a failure rate of less than 1% per year, and normally much, much less. Electronic devices can easily achieve this stringent operational requirement. A new electronic power meter would in particular be designed with proper derating and overall conservatism to guarantee the prescribed long life, target costs and performance.
2.1 Specific Objective of a New Electronic Power Meter
One obstacle to a cost-effective implementation of an all-electronic wattmeter is presented by a particular standard component of an electronic wattmeter: the current sensor transformer.
Consider first that the electronics of any electronic wattmeter require, by definition that they are "electronics" a source of power that is low voltage, i.e., about 5 v.d.c., relative to the power that is monitored, normally 120 v.a.c. or 220 v.a.c. The necessary low-voltage power source requires, when implemented by conventional means, a step-down power transformer in the ratio of approximately eight to one. A minimum of eight plus one, or nine, turns are required in the coil of such an 8:1 step-down power transformer. This requires a component that is typically of some fraction of a U.S. dollar in cost, or as much as the electronic "chips" themselves of the electronic wattmeter cost when produced in mass quantities (circa 1995).
Worse, yet another transformer -- the current sensor transformer -- is required. The electronic wattmeter operational at low voltage clearly cannot directly connect to, or sense, the much higher a.c. voltage the consumption of which is monitored. The power consumption to be monitored, in watts, is the product of load voltage in volts times load current in amperes. The load voltage can be scaled to an appropriate level in a simple and inexpensive resistive divider. However, the load current is determined by a current sensor transformer the primary coil of which is inserted in the path of the input a.c. current.
Responsively to the a.c. current (to the load) in the primary coil of the current sensor transformer, a current is developed in the secondary coil of the same transformer. This current may be dropped across an input resistance of the sensor electronics, developing a voltage that may suitably be interpreted in the electronics as a measure of current to the load. When this sensed current is multiplied by the load voltage then the instantaneous power being delivered to the load is developed. Power consumption in watt hours is simply an integration of this instantaneous power over time.
The current developed in a secondary coil of the current sensor transformer as dropped across the input resistance of the electronic sensor circuit is desirably always less than the d.c. supply voltage on which the sensor electronics are operative, and less than the reference voltage, if the required accuracy of comparison between sensed voltage and reference voltage is to be obtained. This means that the current sensor transformer, like the step-down power transformer, must have a certain minimum number of turns. Moreover, the primary coil of the current sensor transformer is a part of, and is serial in line, the main a.c. power bus (the power consumption of which bus is monitored).
Accordingly, this coil winding must be large and robust, and is typically of heavy gauge copper wire.
According to these requirements, the current sensor transformer is a substantial part that cannot be made as inexpensively as would be desirable, and as would compare favorably with the few pennies cost of other components of the electronic wattmeter, a current sensor transformer costing typically a number of U.S. dollars (circa 1995).
According to the fact that the cost of the two inductive components of an electronic wattmeter commonly equal, or exceed, the costs of the electronic chip components, a design improvement that would eliminate, or minimize, the cost of these inductive components would be desirable.